It is well known that polyester resins, particularly polyethylene terephthalate, polyethylene naphthalate, polytrimethylene terephthalate and polytetramethylene terephthalate have excellent mechanical, physical and chemical properties, and thus are widely utilized for fibers, films and other shaped articles and, particularly, for knitted and woven fabrics, the polyester resin products have excellent mechanical strength, dimensional stability, heat resistance and light resistance.
Each of the above-mentioned polymers for fibers, for example polyethylene terephthalate, is usually produced by, for example, preparing a ethylene glocol ester of terephthalic acid and/or an oligomer thereof, and then polycondensation-reacting the ester monomer or oligomer in the presence of a polycondensation catalyst under a reduced pressure while heating the reaction system until a desired degree of polymerization of the resultant polyester resin is attained. Other polyesters can be produced by procedures similar to the above-mentioned procedures.
With respect to the procedures, it is well known that the quality of the resultant polyester resin is greatly influenced by the type of the polycondensation catalyst and, as a polycondensation catalyst for polyethylene terephthalate, antimony compounds are most widely employed.
However, when an antimony compound is used as the polycondensation catalyst, there arises the following problem. That is, when the resulting polyester is continuously melt-spun for a long time, around a spinneret for melt spinning, foreign matter (hereinafter sometimes merely referred to as spinneret foreign matter) is deposited thereby to cause a bending phenomenon of a molten polymer stream extruded through the spinneret, which leads to the occurrence of fuzz and/or breakage of fiber yarns obtained in the spinning step and/or the drawing step. Particularly, in the production of filaments (of which the performances must be utilized to the maximum extent), the above-mentioned problem must be solved.
To solve the problem, it is known to use a titanium compound, for example, titanium tetrabutoxide as a polycondensation catalyst. In this case, however, the resultant polyester polymer exhibits a low thermal stability and, when melted, the polymer is significantly deteriorated. Therefore the production of the polyester filaments having high mechanical strength is difficult. Also, there arises a problem that the resultant polyester polymer is colored yellow, and the finally resultant fibers exhibit an unsatisfactory color tone.
As means for solving the problem, it is disclosed in, for example, Japanese Examined Patent Publication No. 59-46258, that a product obtained by reacting a titanium compound with trimellitic acid is used as a catalyst for preparation of a polyester, and in, for example, Japanese Unexamined Patent Publication No. 58-38722, that a product obtained by reacting a titanium compound with a phosphite ester is used as a catalyst for producing a polyester. Although the thermal stability of the melt of the polyester is certainly improved to some extent by this processes, the degree of improvement is insufficient and the resulting polyesters have insufficient color tone. Therefore, a further improvement in the color tone of the polyester is required.
Furthermore, Japanese Unexamined Patent Publication (Kokai) No. 7-138354 proposes use of a complex of a titanium compound with a phosphorus compound as a catalyst for the preparation of a polyester. Although the thermal stability of the melt of the polyester is certainly improved to some extent by this process, the degree of improvement is still insufficient and the color tone of the resulting polyester must be further improved.